Color target and method of manufacturing same

ABSTRACT

A semiconductor color target for a single color image pick-up tube incorporated in a single substrate has three different conversion elements having first and second PN-junctions, respectively. The depth of the first junction from the surface is maintained constant regardless of colors, but the depth of the second junction is varied in accordance with the element so as to enable the specified element to have a peak spectrum sensibility to Blue, Green, or Red. Conventional three different color image pick-up tubes can be replaced with a single color tube with the color target according to the present invention.

United States Patent Kubo et al. Jan. 14, 1975 COLOR TARGET AND METHODOF MANUFACTURING SAME Primary Examiner-Richard Murray 75 Inventors:Shuji Kubo; Tohru Itoh, both of hammer-R Mm Gdfrey Kawasaki, Japan [73]Assignee: Matsushita Electric Industrial Co.,

Ltd., Osaka, Japan 57 ABSTRACT [22] Filed: Mar. 16, 1973 A semiconductorcolor target for a single color image [21] Appl' 341896 pick-up tubeincorporated in a single substrate has three different conversionelements having first and [30] Foreign Application Priority Data secondPN-junctions, respectively. The depth of the Mar. 17, 1972 Japan47-27762 first junction from the Surfaee is maintained constant Nov. 10,1972 Japan 47-113361 regardless of colors, but the depth of the Second jtion is varied in accordance with the element so as to 521 US. Cl.358/48, 178/7.1 enable the specified element to have a P spectrum 511Int. Cl. H04n 9/06 Sensibility to Blue, Green, or Conventional three[58] Field of Search l78/5.4 R, 5.4 BD, 5.4 EL, different color image Pp tubes can be replaced 7 7 D 250/211 358/48 with a single color tubewith the color target according to the present invention. [56]References Cited UNITED STATES PATENTS 11/1971 Kato et al. 250/211 .l

10 Claims, 12 Drawing Figures PAIENIEQ A 3,860,956

I SHEET 10F 8 DEPTH FROM SURFACE 5p. lO,u.

PASSING LIGHT WAVE LENGTH m) SHEET 2 BF 8 NFN PATENTEB JAN 1 4l975 SHEET3 BF 8 PATENTED N 1 4 I975 SHEET 5 BF 8 PATENIEUJANMIHYS SHEET 7 OF 8WAVE LENGTH U1.)v

VARTICAL CLOCK 9 VIDIO OUTPUT HORIZONTAL CLOCK COLOR TARGET AND METHODOF MANUFACTURING SAME The present invention relates to a color target,particularly to a target for a color image pick-up tube which does notuse color filters and method of manufacturing same.

In the conventional target for a color image pick-up tube aphoto-electric conductive material is used as a target and non-colorfilters are arranged on the surface at the illuminating side thereof,which pass only Green (G), Red (R), and Blue (B) respectively componentsof incident light. In this manner G, R, and B are converted intoelectrical signals in the corresponding pictures elements and then eachcolor signal is recognized, thus producing color pictures. However,according to this technique there are some difficulties in manufacturingeffective color filters which pass, respectively, only G, R, or Bcomponents and also in arranging a plurality of them in an alignment.

A main purpose of the present invention is to provide a new color imagepick-up tube target in which photoelectric conversion elements sensitiveto G, R, and B components of incident light are used as conversionelements which form the picture elements without necessity of the threecolor filters on a target surface.

An object of the present invention is to provide a color image pick-uptarget without using color filters.

An object of the present invention is to provide a color image pick-uptarget in which photo-electric elements having selective sensibilitiesfor each of R, G, and B components of incident light are formed on asingle semiconductor wafer.

A still another object of the present invention is to provide a colorimage pick-up target which is free from burn on the surface thereof.

A still further object of the present invention is to provide a colorimage pick-up target.

A further object of the present invention is to provide a color imagepick-up tube having the single color target and scanning means forreproducing color signals from the target.

A still further object of the present invention is to provide a methodfor manufacturing the target.

These and other purposes and advantages and features of the presentinvention will become apparent from the following description inconjunction with the accompanying drawings in which:

FIG. 1 shows a diagram showing a relationship between permeability oflight waves and depth from the surface,

FIG. 2(a) shows a fundamental construction of a solid state N-P-Nphoto-electric conversion element according to the present invention,

FIG. 2(b) shows a characteristic of energy band of FIG. 2(a),

FIG. 2(a) shows a fundamental construction of P-N-P photo-electricconversion element according to another embodiment of the presentinvention,

FIG. 2(d) shows a characteristic of energy band of FIG. 2(a),

FIG. 3 shows a spectrum characteristic of the element according to FIG.2(a),

FIG. 4 shows a process of manufacturing a color target according to oneembodiment of the present invention,

FIG. 5 shows a perspective view of the target of FIG. 4 according to thepresent invention,

FIG. 6 shows another process of manufacturing a color target accordingto another embodiment of the present invention,

FIG. 7 shows a characteristic of spectrum of target according to oneembodiment of the present invention,

FIG. 8 shows a color image pick-up circuit according to the presentinvention, and

FIG. 9 shows a block diagram of a scanner for use with a targetaccording to the present invention.

Heretofore, a semiconductor photo-electric conversion element by use ofPN-junction is known. In this element, minority carrier generated byillumination of light reaches an electrode through the PN-junction byturning into a majority carrier, thus obtaining a signal current. Inorder to effectively convert incident light into an electric energy inthe PN-junction semiconductor photo-electric conversion elements, it isnecessary that;

(l) incoming light rays must be effectively projected on to theconversion element so as to produce electronhole pairs, (2) the minoritycarrier produced by the light energy must be passed through the PN-junction without dissipation.

In the meantime the light absorption at the time when a light ray isprojected to a substance depends generally on wavelength of the lightray and a light ray having a short wave length is absorbed in thevicinity of the surface while a light ray having a long wave length isabsorbed at a deep region of the substance.

FIG. 1 illustrates the place where light energy is converted intoelectron-hole pairs, i.e. the condition of absorption with respect toabsorption coefficient of silicon to visible light sensibility. In thefigure, percent of the incoming light energy is absorbed up to thedistance of 5 microns from the surface for the light ray having a wavelength of 0.6 microns. It is to be noted, therefore, that visible lightwith shorter wave length is converted into carrier near the surface ofthe crystal while visible light with longer wave length and the lightrays near the infrared light range are converted into carrier inside ofthe crystal. From this fact, it is noted that by controlling the placewhere the various components of the incident light are absorbed and alsothe place where the carrier resulting from the absorption transverseseffectively across the PN-junction, the photoelectric conversionelements, each of which has a particular peak wave sensibility for Red,Green, or Blue can be made.

In the present invention, a combination of the three differentphoto-electric conversion elements thus produced enables a target tohave particular sensibilities to R, G and B components of the incidentlight.

Now an explanation is made to a photo-electric conversion element whichis sensitive only to Blue, for example. In the element the firstPN-junction is formed in the place near to the surface to which theincoming light is projected, where the conversion takes place. Thesecond PN-junction is formed inside of the crystal, which functions asan internal potential field for removing unnecessary carrier produced bythe light component with a long wave length.

In FIG. 2(a), a fundamental construction of the conversion elementsensitive .to Blue in accordance with one embodiment of the presentinvention is shown. Here, an explanation is made to NPN element. On theN-type silicon substrate 1 are formed a P-type silicon layer 2 havingthickness of 2 microns which is formed by the epitaxial grown method.The N-type silicon layer is formed by diffusion and has thickness ofabout 0.3 microns. The first PN-junction 4 is formed between P- layer 2and N-layer 3, and the second PN-junction 5 is formed betweenN-substrate 1 and P-layer 2. The electrodes 7 and 8 are taken out of thesurface of N-type layer 3 and P-layer 2, respectively. In the element alight ray 6 is directed from the left to the right, so that in this caseN-layer 3 is illuminated. The first PN- junction 4 formed near to thesurface of the element is utilized for a photo-electric conversion.

As it is difficult to form a junction very close to the surface by thepresent semiconductor technique, the components among the visible lightrays below the wave length of 0.6 microns are converted into electricalsignals in the vicinity of the first junction 4, and the components withlong wave length and in the infrared light range are converted at a deepplace passed through the first PN-junction 4. Consequently, most of theconversion for incident light components with a long wave length iscarried out by the minority carrier which is produced at the deep placefrom the surface and is diffused back to the first PN-junction 4 when ittransverses the junction 4. Two electrodes 7 and 8 are provided atP-type layer and N -type layer, respectively.

In this case, in order to prevent the diffused back minority carrierfrom passing through the first PN- junction 4 the second PN-junction 5is provided so that the undesired minority carrier is led to the secondjunction 5 and to reduce the sensibility for the light component with along wave length.

In FIG. 2(b), there is shown a characteristics of energy band of theelement of FIG. 2(a), which has Fermi level 11, conduction band 12, andfilled band 12. The

two depletion layers 9 and 10 are located between the N-layer 3 andP-layer 2, and P-layer 2 and N-substrate l, which correspond to each ofthe layers of FIG. 2(a). In the N-P-N construction, since the electrodes7 and 8 are taken out of the layers 3 and 2, so that only the carrierwhich transverses the depletion layer 9 contributes to a signal current.The minority carrier (holes) which is optically produced in the N-layer1 remains in the filled layer of P-layer 2 and never transverses thedepletion layer 9. In other words, the carrier produced at deeper placesin N-substrate 1 do not contribute to current flow. Only the carrierproduced at the place near to the junction 4 of the P-layer 2 andN-layer 3 contributes to the current. N-P-N construction has beenexplained in the above case, but the same holds true of P-N-Pconstruction.

In FIG. 2(a), there is shown a P-N-P photoelectric conversion element,wherein the same numerals of FIG. 2(a) are used for P-substrate,N-layer, and P-layer with the exception that suffix is added. FIG. 2(d)shows a characteristic of energy band of FIG. 2(0). The two depletionlayers corresponding to those of FIG. 2(b) are also shown.

In order to make an element which is sensitive only to Blue and is notsensitive to Green and Red components of light, the second junction 5should be formed at the place with a distance of 2 to 4 microns from thesurface. The depth of the first junction is made constant within 0.3microns 0.5 microns irrespective of colors to be received. The spectrumcharacteristic of this element is shown in FIG. 3.

Next, in order to make a conversion element which has a maximumsensibility to Green and is not substantially sensitive to Red thesecond PN-junction 5 should be formed at deeper place so as to allow theelement to have much sensibility to the components of light with longwave lengths. In this case the depth of the second junction may be 5 to7 microns from the surface. The depth of the first junction is same asin the case of Blue element as described already.

Likewise, in order to make an conversion element sensitive-to Red, thedepth of the second junction 5 may be 10 to 12 microns from the surface.

As described in the present invention the three different types of theconversion elements, such as the element having a peak spectrumsensibility to Red, the element having a peak to Blue and the elementhaving a peak to Green can be made by varying the distance from thesurface of the element to the second junction. From this fact, a colortarget for a single image pick-up tube can be made by arranging each ofthe three different elements and by incorporating then in a singlesemiconductor substrate.

In FIG. 4, a process for manufacturing the color target is illustrated.A single crystal P-type silicon 20 having 50 d: and 1/ IOOQ-cm isengraved to form different grooves corresponding to each conversiondepths of Green, Red, and Blue. The groove 21 corresponding to Red has adepth of 12 microns, the grooves 22 to Green has a depth of 7 microns,and the groove to Blue has a depth of 4 microns (FIG. 2(a)). The widthof the groove is 15 microns and the pitch thereof is 60 microns.

The crystal .element illustrated in FIG. 4 is for explanation only, sothat the relative length is not exact. The silicon crystal film dopedwith As, namely the epitaxial grown layer 24 having a relativeresistance of OJQ-cmis formed all over the surface by 15 microns (FIG.2(b)). Next, the epitaxial grown layer 24 is removed from all over thesurface 17 microns by a chemical etching and the surface is made flat asmuch as possible (FIG. 2(c)). Accordingly, each of the stripped regions25, 26 and 27, which are epitaxial layers, has 15 microns in widthrespectively. The depth of each of region is 10 microns for Red, 5microns for Green, and 2 microns for Blue. The next process is to coatand oxide silicon film 28 of 3000 A on all over the surface throughthermal-oxiding method (FIG. 2(d)). At the center of each strip theopening 29 of SiO having 5 square microns is formed at the pitch of 15microns by means of photo-resist etching (FIG. 2( e)).

Then, P-region 30 is formed by heating it under boron vapor or boroncomposition vapor at about 1000C and also by diffusing the opening ofSiO, into islands 30 (FIG. 2(f)). The depth of PN-junction is 0.5microns. When the density of the surface is high, the sensibility toshorter wave length tends to deteriorate, so that the boron surfacedensity should be 10 10 cm After removing the boron glass layer which isformed at the time of boron diffusion the electrodes 31, 32 and 33 aretaken out of the N-type regions 25, 26 and 27 which correspond to R, Gand B (FIG. 2(g)) and are connected to the N-type strip.

The aluminium is used for the wire electrodes. Final step is toevaporate a semiconductor 35, by means of, such as trisulfide antimonyon all over the surface 300 A in order to prevent the SiO film fromchanging by electrons emitted due to electron current (FIG. 2(g)).

In FIG. 5, there is shown the target thus produced, where the samenumerals are used.

In FIG. 6, another process of manufacturing the target is shown. In thisprocess, the groove for R has a depth of 8 microns, the groove for G has3 microns and no groove is formed for B (FIG. 6(a)). The width of thegroove is 60 microns and the pitch is 60 microns. The epitaxial grownlayer 24 is formed mircons on all over the surface (FIG. 6(b)).

Next, the epitaxial layer is removed from the surface uniformly as muchas possible by chemical etching, leaving 2 microns of the epitaxiallayer in thickness at the thinest point (FIG. 6(0)). The regionscorresponding to R, G, and B are 10.5 and 2 microns respectively.

Next, a silicon oxide film 28 is formed about 3000 A on all over thesilicon surface by thermal oxiding method (FIG. 6(d)). The Si0 openingin the form of square of 30 microns is formed at the center of eachstrip (FIG. 6(e) The opening is formed by photo etching method. Thesilicon opening is further heated under boron vapor at the temperatureof 1000C and boron is diffused into the opening and P-type regions 30are formed (FIG. 6(f)). The depth of the junction is 0.5 microns. Whenthe diffused density of the surface is high the sensibility. to thecomponents of light having a shorter wave length is deteriorated, sothat the boron surface density must be 10 l0 cm.

The next process is to remove a silicon oxide film including boron glassby a chemical etching method. After that a silicon oxide film 31 isgrown 2000 A on all over the surface of the substrate by heat-oxidingmethod (FIG. 6(g)). Then, a plurality of holes are provided on thesilicon oxide film of the P-type final island regions 30-E and theoutput terminal electrode 32 and the charge transfer electrode 33 areprovided by photoresist etching (FIG. 6(h)).

In the foregoing example, the distance from the crystal surface to thesecond junction is changed so as to give each of the elements of R, Gand B aparticular sensibility to the colors and the N-type regions 25,26 and 27 should be made different respectively through chemicaletching, epitaxial method, or chemical etching techniques. Namely,aluminium is diffused on a flat P-type substrate on the regioncorresponding to Green, and boron is selectively diffused on the regionto Blue. No diffusion is made to Red element. N-type epitaxial is grownthereon. During the epitaxial growing aluminium and boron in the regionsare diffused into the epitaxial layer. Aluminium is faster than boron indiffusion speed. The distance from the surface of grown layer to thefirst junction and each of the growing times are defined as follows;

10 microns for Red 5 microns for Green 2 microns for Blue The peak ofthe X-cell spectrum sensibility resides in the light wave length of 0.45and the sensitivity is 0.034 ptA/uW cm. The peak of the Y-cell spectrumsensibility resides in 0.55 micron and the light sensitivity is 0.062uA/uW cm.

The light sensibility of Y-cell at the light wave length of 0.45 micronsis approximately equal to that of X cell of 0.45 microns-wave length.The peak of Z-cell spectrum sensibility resides in 0.65 microns and thelight sensibility thereof is 0.l;LA/p.W cm. The light sensibility of thetwo cells for 0.45 microns and 0.55 microns is equal to the lightsensibility of X cell and Y cell. In

FIG. 7, there is shown a characteristic of spectrum sensivilites of thethree different elements X, Y and Z, each having a maximum spectrumsensibility for Red, Green and Blue. Accordingly, the relationshipexpressed by the following equations.

Z=R+G+B Y=G+B X=B (1) Therefore, each component of R, G and'B isexpressed by the following equations from the spectrum light sensibilitycharacteristics of X, y and Z.

With respect to the light sensibility of the silicon P-N junction diode,the following equation is theoretically established.

However, the actual light sensibility of the diode measured was turnedout to be the following.

The reason for this will be derived from the followl. The shorter thewave length becomes, the larger the reflective efficiency of siliconsurface becomes.

2. The shorter the wave length of light becomes, the more the minoritycarrier is generated at the place near to the surface and the larger theprobability of extinction due to recombination at the surface becomes.

Accordingly, in order to allow equation (4) to be approximate toequation (3), the following process is required;

1. An anti-reflection film is evaporated on the silicon surface. Forexample, SiO is coated by about 500 A.

2. Crystal defects should not be made on'thelight incoming surface ofsilicon.

When each of the X, Y and Z-cell on a single silicon substrate isscanned by electron beam, X, Y and Z cells are sampled and correspondingoutputs are produced and an arithmetic operation as shown in equation(2) is carried out in an arithmetic circuit. When R, G and B have anequal light intensity, equation (4) is established, so that it isnecessary to adjust the signals to adapt to human visual sensibility.

Accordingly, the circuit for correcting the signals should carry out anoperation including R, aG, 88, where a and B are coefficientrespectively.

Referring to FIG. 8, there is shown one embodiment of the single colorimage pick-up tube with the target. In the figure, the image pick-uptube 40 is an iconoscope type tube which comprises the color target 41which is scanned by electron beam emitted from the cathode 42 anddeflected by the well-known technique in accordance with the scanningframe line. Since the cathode 42 is suitably displaced in a position theimage 43 is passed through the lens 45 and through the transparentportion 45 of the tube to reach the target 41 to be scanned by electronbeam, where the image is directly focused.

The carriers in the target as the result of illumination are taken outas an electric current when the beam reached the target and the outputvoltages are generated across the output resistor 47, 48 and 49. Theoutput voltages are taken out from the terminals 50, 51, 52 as X signal,Y signal, and Z signal respectively. Numeral 53 shows a bias source.

In the foregoing description reference is made to the case in whichlight is projected from the first junction. However, when light isintroduced from the second junction or substrate the characteristic ofvoltage becomes the one in which the shorter wave length is cut andX-cell comes to include. Signal having R, G and B while Y-cell comes toincludes a signal having R and G, and Z cell includes only R.

FIG. 9 shows blockdiagram of a scanning means having a photo-electricconversion matrix 71 such as shown in FIG. 5, vertical scanning signalgenerator 72, transfer gate 73 output resistor 74 and output amplifier75. When the electric charges accumulated in the matrix are desired tobe transferred, the generator 72 operates the particular transfer gate73 to be scanned, and the charges are transferred to output resistor 74.In this case, when horizontal clock pulses are applied the charges aremoved successively into output amplifier 75.

It is to be noted that the effects and advantages according to thepresent invention will be;

1. The color image pick-up tube can be made a single tube, (2) the colorfilters and signal index can be dispensed with, (3) the target is easilymanufactured by the present integral circuit techniques such as siliconplanar technique, so that the target is economical and is suitable for amass production as well as it has a good stability, (4) since the targetis a single crystal, spot is prevented from burning out and value of yof image is nearly 1.

It is to be noted that the present invention is not to be limited to theexact construction shown and described and that various changes andmodifications may be made without departing from the spirit and scope ofthe invention.

What is claimed is:

l. A semiconductive photoelectric converting device comprising asemiconductive substrate of one conductivity type having alight-receiving surface, a plurality of separate first p-n junctionsjuxtaposed in said substrate at a predetermined depth from said surface,and a plurality of separate second p-n junctions equal in number as saidfirst p-n junctions and spaced therefrom at different depths from saidsurface corresponding to the red, green and blue components respectivelyof light incident on said surface.

2. The device as claimed in claim 1, wherein said semiconductivesubstrate is silicon.

3. The device as claimed in claim 2, wherein said predetermined depth isfrom 0.3 to 0.5 microns.

4. The device as claimed in claim 2, wherein said different depths rangefrom 2 to 12 microns.

5. The device as claimed in claim 1, wherein said light-receivingsurface is coated with a film of silicon dioxide.

6. A television camera tube comprising an evacuated envelope, afaceplate at one end thereof, an electron gun at the other end toprovide an electron beam towards said faceplate, a semiconductivephotoelectric converting device as claimed in claim 12 mounted on theinner surface of said faceplate, means coupled to said device andderiving electrical signals when the carriers generated at differentdepths from the light receiving surface of said device by the lightincident thereon traverse said p-n junctions thereof, said electricalsignals including a first signal corresponding to the full lightwavelength range of visible spectrum, a second signal corresponding totwo of the primary color components in said wavelength range and a thirdsignal corresponding to one of said two color components, a firstsubtracting circuit for subtracting said second signal from said firstsignal to derive a first color signal, and a second subtracting circuitfor subtracting said third signal from said second signal to derive asecond color signal, said third signal being a third color signal.

7. A method for fabricating a photoelectric converting device as claimedin claim 1, comprising the steps of forming parallel grooves atdifferent depths corresponding to the red, green and blue componentsrespectively of light into a semiconductive substrate of oneconductivity type, growing an epitaxial layer on said substrate, etchingsaid layer to provide a uniform surface, coating a film of silicondioxide on said surface, etching said silicon dioxide film to provide aplurality of windows, and diffusing boron through said windows into saidepitaxial layer.

8. The method as claimed in claim 7, wherein said grooves have a depthof about 4 microns for the blue component, a depth of about 7 micronsfor the green component and and depth of about 12 microns for the redcomponent.

9. The method as claimed in claim 7, wherein said boron is diffused at atemperature of about 1000C.

10.The method as claimed in claim 7, wherein said parallel grooves havea step-like shape in cross section.

1. A semiconductive photoelectric converting device comprising asemiconductive substrate of one conductivity type having alight-receiving surface, a plurality of separate first p-n junctionsjuxtaposed in said substrate at a predetermined depth from said surface,and a plurality of separate second p-n junctions equal in number as saidfirst p-n junctions and spaced therefrom at different depths from saidsurface corresponding to the red, green and blue components respectivelyof light incident on said surface.
 2. The device as claimed in claim 1,wherein said semiconductive substrate is silicon.
 3. The device asclaimed in claim 2, wherein said predetermined depth is from 0.3 to 0.5microns.
 4. The device as claimed in claim 2, wherein said differentdepths range from 2 to 12 microns.
 5. The device as claimed in claim 1,wherein said light-receiving surface is coated with a film of silicondioxide.
 6. A television camera tube comprising an evacuated envelope, afaceplate at one end thereof, an electron gun at the other end toprovide an electron beam towards said faceplate, a semiconductivephotoelectric converting device as claimed in claim 12 mounted on theinner surface of said faceplate, means coupled to said device andderiving electrical signals when the carriers generated at differentdepths from the light receiving surface of said device by the lightincident thereon traverse said p-n junctions thereof, said electricalsignals including a first signal corresponding to the full lightwavelength range of visible spectrum, a second signal corresponding totwo of the primary color components in said wavelength range and a thirdsignal corresponding to one of said two color components, a firstsubtracting circuit for subtracting said second signal from said firstsignal to derive a first color signal, and a second subtracting circuitfor subtracting said third signal from said second signal to derive asecond color signal, said third signal being a third color signal.
 7. Amethod for fabricating a photoelectric converting device as claimed inclaim 1, comprising the steps of forming parallel grooves at differentdepths corresponding to the red, green and blue components respectivelyof light into a semiconductive substrate of one conductivity type,growing an epitaxial layer on said substrate, etching said layer toprovide a uniform surface, coating a film of silicon dioxide on saidsurface, etching said silicon dioxide film to provide a plurality ofwindows, and diffusing boron through said windows into said epitaxiallayer.
 8. The method as claimed in claim 7, wherein said grooves have adepth of about 4 microns for the blue component, a depth of about 7microns for the green component and and depth of about 12 microns forthe red component.
 9. The method as claimed in claim 7, wherein saidboron is diffused at a temperature of about 1000*C.
 10. The method asclaimed in claim 7, wherein said parallel grooves have a step-like shapein cross section.